1. Field of the Invention
This invention relates to a support for a lithographic printing plate and an original forme for the lithographic printing plate (hereinafter also abbreviated as lithographic printing original plate for ease of description). More specifically, the present invention relates to: an original forme for a heat-sensitive lithographic printing plate, which may be used suitably for the computer-to-plate system that does not require the development process, in particular a heat-sensitive lithographic printing plate which may be image-recorded by means of infrared-scanning exposure based on digital signals and may be mounted directly to a printing machine to perform printing without performing the conventional development process using a developer after exposure; and a support for the lithographic printing plate which is used therefore. Also, it relates to: an original forme for a heat-sensitive lithographic printing plate, which is printable after performing a conventional development process using a developer after the exposure; and a support for the lithographic printing plate which is used therefore.
2. Description of the Related Art
There have been many studies on a lithographic printing original plate used for the computer-to-plate system which has developed remarkably in recent years. In particular, aiming at a further rationalized printing process and the solution for the problem of waste liquor disposal, a number of studies have been conducted and a variety of proposals have been made as to a lithographic printing original plate which may be directly mounted to a printing machine after the exposure to carry out printing.
As a promising technique, there is a lithographic printing original plate which comprises as a heat-sensitive image-forming layer of a water receptive layer in which particles of hydrophobic thermoplastic polymer are dispersed in a hydrophilic binder polymer. This lithographic printing original plate utilizes the principle such that, when the heat-sensitive layer is heated, the particles of hydrophobic thermoplastic polymer are fused together so that the surface of a hydrophilic heat-sensitive layer is changed to an ink-receptive image area.
For such a lithographic printing original plate utilizing the thermal fusion of particles of hydrophobic thermoplastic polymer, as one means to reduce the number of treatment processes, there is a method called on-press development wherein the lithographic printing original plate after the exposure is mounted to the cylinder of a printing machine without performing thereon a treatment with developer solution and then nonimaged areas of the original forme is removed by feeding ink and/or damping solution while rotating the cylinder, or a lithographic printing original plate of a non-treatment type in which the development is not performed. In the latter method, the lithographic printing original plate is directly mounted to a printing machine after performing the exposure and printing is completed without performing the development according to a conventional printing process.
Such a lithographic printing original plate, which is suitable for the on-press development or non-treatment type printing plate, includes a heat-sensitive layer soluble in a damping solution or ink solvent and is required to have handleability under light which is suitable for performing the development on a printing machine placed in a light room.
For example, JP 2,938,397 B describes a lithographic printing plate wherein a heat-sensitive layer, in which fine particles of thermoplastic hydrophobic polymer are dispersed in a hydrophilic binder polymer, is formed on a hydrophilic support. In the specification thereof, it is described that the on-press development can be carried out by exposing the above-mentioned lithographic printing original plate to an infrared laser to thereby unite fine particles of thermoplastic hydrophobic polymer by heat to form an image, and then mounting it to a cylinder of a printing machine and feeding ink and/or damping solution.
JP 9-127683 A and WO 99/10186 describe the production of a lithographic printing plate by uniting thermoplastic fine particles by heat and then performing the on-press development.
Such a lithographic printing original plate, on which an image is formed by the uniting of fine particles by heat, exhibits a good on-press developability. However, it has such problems as reduced sensitivity due to loss of heat to the metal support, and an insufficient press life due to reduced hardness of an image area on the heat sensitive layer in the case where the uniting of the fine particles was insufficient.
As a countermeasure to these problems, a method has been proposed, in which a water-insoluble organic polymer is provided between an aluminum support and a heat-sensitive layer (for example, JP 2000-23983 A). Although this method provides higher sensitivity, it has a problem in that contamination occurs.
Incidentally, examples of the conventional thermal-type lithographic printing plates in which on-press development is not performed include: the so-called thermal type positive lithographic printing original plate wherein an infrared absorbent in the heat-sensitive layer expresses it photothermal conversion action and generates heat upon the exposure and the exposed areas of the heat-sensitive layer are alkali-solubilized with the heat thus generated to form a positive image; and the thermal type negative lithographic printing original plate wherein a radical-yielding agent and an acid-generating agent respectively generate radicals and acids with the heat thus generated, whereby the radical polymerization and acid bridging reactions proceed to insolubilize the exposed areas to thereby form a negative image. However, these have the problems as mentioned below.
That is, in such an image formation process of a thermal type, a heat is produced by a photothermal conversion in the heat-sensitive layer by means of laser beam and the thus produced heat causes a reaction for the image-formation. Here, in the aluminum support whose surface has been roughened and on which an anodized film is formed, the heat conductivity of the support is extremely higher than that of the heat-sensitive layer, so that the heat generated in the vicinity of the interface between the heat-sensitive layer and the support is dispersed into the inside of the support before it is adequately used for the formation of image. As a result, the following problems occur on the interface between the heat-sensitive layer and the support.
First, with respect to the positive type heat-sensitive layer, there is a problem of low sensitivity. That is, when the heat is dispersed into the inside of support to cause an insufficient alkali-solubilization reaction, a film remains on otherwise nonimage areas, which is regarded as an essential problem of a positive type heat-sensitive layer.
Further, in such a lithographic printing original plate of thermal positive type, an infrared absorbent having a photothermal conversion function must be provided. Such an infrared absorbent has a low solubility due to its relatively high molecular weight, and is difficult to remove since it is adsorbed onto the micro-openings formed through the anodic oxidation, so that during the development process using the alkaline developer, the problem of residual films can easily occur.
On the other hand, in the negative type heat-sensitive layer, when the heat is dispersed into the inside of support to cause insufficient developer-insolubilization of the heat-sensitive layer in the vicinity of the interface between the heat-sensitive layer and the support, an image may not be formed in a satisfactory manner in an area that should be the image area and is carried away during the development, or even if an image is formed, it may be easily peeled off during the printing.